1. Field of the Invention
The present invention relates to a projection exposure apparatus which, for example, is used in the process of manufacturing semiconductor devices. More particularly, it relates to a so-called step-and-scan type projection exposure apparatus for scanning a mask and a photosensitive substrate synchronously in one-dimension at the time each of shot areas on the photosensitive substrate is exposed.
2. Related Background Art
Up to this time, in manufacturing semiconductor devices or liquid crystal display devices, etc. under photolithography technique, projection exposure apparatuses have been utilized in which a pattern formed on a reticle (or a photomask, etc.) is exposed via a projection optical system on a wafer (or a glass plate, etc.) with photoresist or the like applied thereto. In such projection exposure apparatuses, step-and-repeat type (batch exposure type) projection exposure apparatuses have been principally utilized in which a reticle and a wafer are exposed under a stationary state thereof after setting each of shot areas provided on the wafer to the exposure position by stepping actuation of a wafer stage like a stepper.
Generally, with the projection exposure apparatuses, it is required to keep image forming performance of the projection optical system (aberration caused by heat deformation of a lens absorbing exposure light, defocus or the like) within an allowable range, so that a mechanism for adjusting the image forming performance (also including its measurement) is provided. For example, an image forming performance adjusting device is used, which includes a mechanism for adjusting gaseous pressure of hermetically sealed spaces between groups of lenses constituting the projection optical system according to the exposure amount, a mechanism for adjusting a predetermined position or inclined angle of lenses constituting the projection optical system, or the like. Also, an oblique incidence type focal position detecting device or the like is used for measuring a degree of defocus, in which an image of slit-pattern is projected relative to the exposure surface of the wafer so that the focal position of the wafer is measured based on the position of the image formation of the reflected light.
With the recent trend toward increasing the size of a chip pattern of a semiconductor, a projection exposure apparatus is required to expose a larger area of the pattern of the reticle onto the wafer. Therefore, a projection exposure apparatus of a so-called step-and-scan type or a slit scan type (scan type exposure apparatus) has been proposed in which the reticle is illuminated with a rectangular-shaped or circular-shaped illumination area (hereinafter, called the "slit-shaped illumination area") and the reticle and the wafer are synchronously scanned with respect to the projection optical system so as to expose the pattern of the reticle.
Even in such a scan type exposure apparatus, it is required to keep image forming performance of the projection optical system within an allowable range. However, a conventional mechanism for the batch exposure type projection exposure apparatus has still been utilized for adjusting the image forming performance. Also, in the scan type exposure apparatus, although it is required to include an alignment device for positioning each of the shot areas on the reticle and the wafer, and a focus leveling device for positioning the photosensitive substrate in the direction of the optical axis of the projection optical system, the conventional mechanism for the batch exposure type projection exposure apparatus has been similarly utilized for the alignment device and the focus leveling device.
FIG. 14 shows a conventional scan type projection exposure apparatus which includes such a mechanism as a sensor or the like. In FIG. 14, an alignment microscope 27 of through-the-lens type (TTL) is provided in the neighborhood of an upper end of a projection optical system 14 for detecting positions of alignment marks of the respective shot areas on a photosensitive substrate 17 via the projection optical system 14. Also, another alignment microscope 28 is provided in the neighborhood of the lower end of the projection optical system 14 for detecting the alignment marks on the photosensitive substrate 17 according to an off-axis system. The alignment microscopes 27 and 28 position the photosensitive substrate 17.
Then, a through-the-lens type focus leveling sensor (not shown) for positioning the photosensitive substrate 17 in the direction of the optical axis of the projection optical system 14, i.e., for focussing and leveling, or a so-called oblique incidence type focus leveling sensor placed in a space between the projection optical system 14 and photosensitive substrate 17 are provided. FIG. 14 only shows a floodlight system 29 of the oblique incidence type focus leveling sensor. In addition, although the alignment microscope 28 and the floodlight system 29 are practically placed in asymmetric positions relative to the optical axis of the projection optical system 14, they are shown on the same plane in FIG. 14 for convenience.
The sensor for positioning means is required to be out of contact, so that an optical sensor is chiefly used. When performing accurate measurement with such an optical sensor, it is important to avoid air fluctuation of an optical path of probe light as much as possible. For this, an exclusive air conditioning device 130 can be provided between a reticle 6 and the projection optical system 14.
Further, in FIG. 14, an actuator 131 is attached for actuating, for example, a lens element 15 out of lens elements of the projection optical system 14. The position or the inclined angle of the lens element 15 is finely adjusted through the actuator 131 so that some aberrations of the projection optical system 14 are adjusted, thereby obtaining abetter projected image.
Lens elements other than the lens element 15 can be actuated by the actuator 131 inside the projection optical system 14.
As described above, in the conventional scan exposure type projection exposure apparatus, the batch exposure type mechanism is utilized for adjusting the image forming performance (including its measurement). Also, the batch exposure type mechanism is utilized as the alignment device for the conventional scan exposure type projection exposure apparatus. However, the scan exposure type projection exposure apparatus tends to make the allowable range more narrow with respect to the image forming performance of the projection optical system being in a stationary state. Accordingly, it has been desirable to provide a more highly accurate mechanism for adjusting the image formation.
Similarly, it has been desirable to improve the alignment mechanism used for the scan exposure type projection exposure apparatus. For the purposes of this specification, the alignment mechanism is to be considered in a broad sense as to constitute a kind of adjusting mechanism for adjusting image forming performance of the projection optical system. Also, an image forming property, such as distortion or curvature of field, is to be considered as a kind of image forming performance.
In the conventional scan type projection exposure apparatus, various mechanisms (an air conditioning mechanism, an alignment mechanism and the like) are mounted around the projection optical system so as to secure very high accuracy. As a result, the mounted mechanisms may mechanically interfere with each other under certain circumstances, which presents a problem in that the system can be difficult to design or install, or in that a desired performance cannot be obtained because of an unreasonable arrangement.